Generally, the afterglow time of a phosphor is extremely short, i.e., the light emitted from the phosphor rapidly decays after external excitation is stopped. However, after being excited with ultraviolet radiation or the like, some phosphors rarely show afterglow which can be perceived by the naked eye for a considerable time (ranging from several tens of minutes to several hours) even after the excitation is stopped. Such phosphors are called phosphorescent phosphors or phosphorous materials as distinguished from normal phosphors.
Sulfide phosphors such as CaS:Bi (which emits violet blue light), CaSrS:Bi (which emits blue light), ZnS:Cu (which emits green light) and ZnCdS:Cu (which emits yellow or orange light) are known as such phosphorescent phosphors. However, any of these sulfide phosphors has many problems from a practical viewpoint, e.g., chemical instability, poor photo-resistance. Moreover, even when this zinc sulfide phosphor is used for a luminous clock, afterglow allowing the time to be perceived by the naked eye lasts only about 30 minutes to 2 hours.
Thus, the applicant invented a phosphorescent phosphor comprising a compound expressed by MAl2O4 as a matrix, in which M is at least one metal element selected from the group consisting of calcium, strontium and barium, as a phosphorescent phosphor which has afterglow characteristics that last much longer than those of commercially available sulfide phosphors, chemical stability and excellent photo-resistance for a long time, and obtained a patent thereof (see Japanese Patent No. 2543825 Publication: patent document 1).
Owing to the invention of the aluminate phosphorescent phosphor described in the patent publication, it has become possible to provide a long-afterglow phosphorescent phosphor which has afterglow characteristics that last much longer than those of conventional sulfide phosphors, chemical stability due to an oxide, excellent photo-resistance and broad applicability to various usage.